This invention relates to thermoplastic pipe. In particular, the invention relates to apparatus which can be used for forming bends in thermoplastic pipe. The invention also relates to a method of forming a bend in thermoplastic pipe, which method utilises the apparatus of the invention.
Thermoplastic piping is now widely used for water and sewage reticulation and for gas supply. Such piping can be supplied as discrete lengths but in some instances is provided as rolls allowing continuous laying of pipe over long distances.
Irrespective of the length of the sections used to make up a piping system, bends are invariably required. This is despite the fact that thermoplastic pipe, particularly of smaller wall thicknesses, has some flexibility. Bends are typically provided at angles of 30xc2x0, 45xc2x0, 60xc2x0 or 90xc2x0.
To be incorporated into the piping system the bends must be connected to the adjacent pipe sections in the same fashion as the pipe sections per se must be joined. Thermoplastic pipe sections are commonly connected by electrofusion or butt welding. In some cases, a butt flange may be provided between sections but each half of the butt flange must be connected to a section using the foregoing methods.
So that a join can be formed that is commensurate with the pressure rating of the pipe, it is important that the ends of pipe sections to be joined are both circular. That is, if one section has a circular end while the other end has a slightly oval end, an inferior joint will be produced.
According to present practice, bends are formed in sections of thermoplastic pipe by supporting a section of softened pipe at its ends and applying pressure at the centre of the section to form the desired bend.
A disadvantage of the known method for forming bends in thermoplastic pipe is that the pipe is stretched preferentially in the middle of the arc during the bending process. This is particularly the case with the outside wall of the bend. The stretching weakens the pipe wall and leads to derating of the pressure rating of the bend.
The object of the present invention is to provide a method of forming a bend in a section of thermoplastic pipe, which method overcomes the disadvantages of the existing method for forming such bends.
In one aspect, the invention provides apparatus for forming a bend in a linear section of thermoplastic pipe, the apparatus comprising:
an anvil having a concave working surface which defines an arc of at least the bend angle of the bend to be formed in said pipe section and which is transversely arced at a radius essentially the same as the radius of said pipe, said anvil further including a collar for retaining an end of said pipe section adjacent said working surface; and
a flexible mandrel of substantially the same internal diameter as said pipe and of at least the same length as said pipe section.
In a second aspect, the invention provides a method of forming a bend in a linear section of thermoplastic pipe, the method comprising the steps of:
(i) heating said pipe section to the softening temperature thereof;
(ii) driving a flexible mandrel of substantially the same internal diameter as said pipe and of at least the same length as said pipe section into said pipe section;
(iii) loading said pipe section and mandrel onto an anvil having a concave working surface which defines an arc of at least the bend angle of the bend to be formed in said pipe section and which is transversely arced at a radius essentially the same as the radius of said pipe, said anvil further including a collar for retaining an end of said pipe section adjacent said working surface;
(iv) drawing the end of said pipe section opposite said retained end towards said working surface to bring the bulk of the pipe section into contact with said working surface;
(v) allowing said pipe section to cool to below said softening temperature while held in contact with said working surface; and
(vi) withdrawing said mandrel from said pipe section.
As used herein, the terms xe2x80x9ccomprisingxe2x80x9d and variants thereof such as xe2x80x9ccomprisexe2x80x9d and xe2x80x9ccomprisedxe2x80x9d denote that the described apparatus or method includes the stated integer or integers but do not necessarily exclude the inclusion of other integers.
The term xe2x80x9cthermoplasticxe2x80x9d as used herein applies particularly to polyolefins such as polyethylene and polypropylene.
It will be appreciated from the two aspects of the invention defined above that the anvil provides a former for the bend while the mandrel maintains the circularity of the pipe section, particularly at the ends of the section.
The anvil working surface is in effect a curved channel which receives the softened pipe section. As noted above, the radius of the transverse channel arc is essentially the same as the radius of the pipe. This aids maintenance of the circularity of the pipe during the bending process.
The angle of the arc defined by the anvil working surface can be equivalent to the desired bend angle of the pipe section. Preferably, however, the arc has an angle of about 100xc2x0. To provide for bend angles of less than 90xc2x0 using the preferred anvil, the retaining collar is adapted to be repositionable. Thus, if a bend angle of only 30xc2x0 is required, the retaining collar is repositioned to give an arc of about 35xc2x0. The retaining collar is typically repositionable throughout the range of 30xc2x0 to 90xc2x0.
To facilitate use of the anvil of apparatus according to the first aspect, the anvil is advantageously mounted to a support frame, which frame can be secured to a supporting surface such as a workshop floor or the like.
For use with pipe of greater than 125 mm diameter, apparatus according to the first aspect advantageously includes a stepped track for supporting the softened pipe section and the mandrel. A lower portion of the track is for supporting the pipe while a higher portion supports the mandrel. The difference between the lower and higher portions of the track is roughly equivalent to the wall thickness of the pipe. This allows the mandrel to be slid from its supporting section of track directly into the bore of the pipe section. The track advantageously comprises a pair of spaced apart rail members with a support frame.
In a particularly preferred embodiment of the invention, a trolley is provided on the upper portion of the track on which trolley the mandrel is carried. The trolley is advantageously height adjustable so that different wall thicknesses of the pipe section can be compensated for when the mandrel and pipe section axes need to be aligned.
With apparatus including a stepped track, the anvil is pivotally mounted to the support frame and positioned adjacent the lower portion end of the track with the working surface of the anvil aligned with the track. For loading the softened pipe section, the anvil is pivoted towards the track to present the retaining collar to the track. Once the collar has received the pipe end, the anvil can be pivoted away from the track so that the pipe section can be brought into contact with the working surface.
Turning to the mandrel of apparatus according to the first aspect of the invention, this typically comprises a coil spring within a sleeve of an elastomeric material such as rubber. Sizing rings can be provided at ends of the mandrel. The sleeve can be stepped to receive sizing rings.
Apparatus according to the invention can be used for forming bends in thermoplastic pipe sections of any diameter including diameters of 90 mm up to 1,000 mm. It will be appreciated, however, that apparatus components must be configured appropriately for the pipe section diameter.
In the method according to the second aspect, the section of thermoplastic pipe can be softened using any of the methods known to those of skill in the art. Preferably however, the pipe section is heated in a thermostatically-controlled bath of a non-volatile liquid such as glycerine. Rollers can be included in the bath to ensure even heating of the pipe.
To aid insertion of the mandrel into the softened pipe section in step (ii) of the method, the mandrel is advantageously lubricated. A suitable lubricant is liquid soap.
Steps (ii) and (iii) of the method can be mechanically aided. For example, a winch can be used to drive the mandrel into the pipe section. A winch can also be used for steps (iii) and (iv). Pivoting of the anvil is typically aided by a hydraulic ram.
Withdrawal of the mandrel in step (vi) of the method according to the second aspect can be mechanically aided such as by using a winch.